1. Field of the Invention
The present invention relates to a semiconductor integrated circuit that matches the resistance of a variable resistor, which is used as a terminating resistor or as a reference of the terminating resistor, to the characteristic impedance of a transmission line.
2. Background Art
Conventionally, in order to maintain the quality of the waveform of signals in the high-speed signal transmission at frequencies of several hundreds of MHz or higher, for example, it is highly important to precisely control the resistance of a terminating resistor to provide impedance matching to a transmission line.
However, the resistance of the terminating resistor varies with the semiconductor manufacturing process, the type of the power supply, and the temperature, for example. Thus, to control the resistance of the terminating resistor with precision to ±5% to ±10%, for example, the semiconductor integrated circuit has to have a terminating resistor adjusting circuit for accommodating the variations in resistance of the terminating resistor caused by the factors described above.
As an example, a conventional semiconductor integrated circuit has a terminating resistor adjusting circuit that includes a current circuit connected to a power supply, a variable resistor connected between the current circuit and the ground to receive a main current output from the current circuit, a comparator circuit that compares the potential of the variable resistor with a first reference potential and outputs a signal, and a control circuit that controls the resistance of the variable resistor based on the output signal of the comparator circuit (see Japanese Patent Laid-Open No. 2004-336699).
In this way, the conventional semiconductor integrated circuit described above controls the resistance of the terminating resistor taking into account variations in factors relating to the silicon substrate itself on which the semiconductor integrated circuit is formed (the manufacturing process, the power supply and the temperature, for example).
Even if the resistance of the terminating resistor is precisely adjusted taking into account variations in factors relating to the silicon substrate itself, external parasitic resistances due to components external to the silicon substrate including the test element group (TEG), the package, which varies with the product, the printed circuit board (PCB), that is, external parasitic resistances existing between the terminating resistor and the transmission line are actually added to the adjusted resistance of the terminating resistor when the product is completed.
However, the semiconductor integrated circuit has no mechanism to accommodate the variations in resistance due to the external parasitic resistances.
Therefore, the conventional semiconductor integrated circuit described above has a problem that the adjustment capability of the terminating resistor adjusting circuit has to be adjusted for each TEG or product because of the offset from the characteristic impedance of the transmission line due to the external parasitic resistances.